Special counsel Robert Mueller’s report revealed new details about the investigation of former national security adviser Michael Flynn.

The FBI investigated Flynn’s possible ties to Russia earlier than previously known, according to the report.

Mueller was also authorized to investigate Flynn over four separate matters.

The FBI was investigating Michael Flynn’s possible relationship with the Russian government much earlier than previously known, the special counsel’s report revealed.

Flynn, who served as President Donald Trump’s national security adviser, was also under investigation for four separate sets of allegations, says the report, which was released Thursday.

It was already known that Flynn was under investigation over phone calls he had with Russian Ambassador Sergey Kislyak in late December 2016, during the presidential transition period. Flynn pleaded guilty in the special counsel’s investigation on Dec. 1, 2017 for making false statements about those phone calls.

But special counsel Robert Mueller’s report reveals for the first time that Flynn was a target of the FBI’s Russia probe before the Kislyak calls.

“Previously, the FBI had opened an investigation of Flynn based on his relationship with the Russian government,” reads the report, which cited FBI interviews given by former Justice Department official Mary McCord and former FBI Director James Comey. (RELATED: Mueller Report ‘Inconclusive’ That Trump Knew About Flynn Call With Kislyak)

“Flynn’s contacts with Kislyak became a key component of that investigation,” it said.

It is unclear what would have prompted scrutiny of Flynn’s ties to Russia prior to his Kislyak calls.

Flynn was reportedly one of the four Trump associates initially placed under FBI investigation.The bureau opened a counterintelligence investigation, code-named “Crossfire Hurricane,” on July 31, 2016, after receiving a tip from the Australian government regarding Trump campaign aide George Papadopoulos.

Special counsel Robert Mueller (L) arrives at the U.S. Capitol for closed meeting with members of the Senate Judiciary Committee June 21, 2017 in Washington, D.C. (Photo by Alex Wong/Getty Images)

In congressional testimony in December, Comey said that four individuals were initially placed under investigation. It is still not clear if the file against Flynn was opened at the same time as Papadopoulos’s or if that happened later.

One area of investigation may have been Flynn’s visit to Moscow in December 2015. The retired lieutenant general was paid $45,000 to attend an event hosted by RT, the Russia-owned news outlet. Flynn sat at the same table as Russian President Vladimir Putin.

It has also been reported that American intelligence authorities were provided a tip regarding Flynn’s interactions in February 2014 with a Russian-British academic at Cambridge University.

Flynn served as director of the Defense Intelligence Agency at the time of the visit to Cambridge, England. The event was hosted by Sir Richard Dearlove, the former head of MI6, and Stefan Halper, an academic who has been identified as a longtime FBI and CIA informant. Dearlove and Halper grew concerned with Flynn’s interactions with the Cambridge academic, Svetlana Lokhova, according to The Washington Post. Those concerns were conveyed to American and British intelligence, though it is not certain when that occurred.

Lokhova has pushed back aggressively on reporting that has suggested anything improper transpired during her interactions with Flynn.

The fact that Flynn was under investigation for four sets of allegations was also not previously known.

Deputy Attorney General Rod Rosenstein included the Flynn matters in a memo to the special counsel on Aug. 2, 2017 that authorized various components of the investigation, according to the Mueller report.

Rosenstein authorized Mueller to investigate allegations that Trump campaign associates Carter Page, Paul Manafort and Papadopoulos “committed a crime or crimes by colluding with Russian government officials with respect to the Russian government’s efforts to interfere with the 2016 presidential election.”

Rosenstein also authorized Mueller to investigate Manafort’s Ukraine business activities, as well as whether Papadopoulos was working as an unregistered foreign agent of the Israeli government.

“Four sets of allegations involving Michael Flynn, the former National Security Advisor to President Trump,” were also authorized for investigation, Mueller’s report says.

Manafort was convicted in federal court on charges related to his Ukraine work. Mueller found no evidence that any of the Trump advisers worked with Russia to influence the election.

In addition to his calls with Kislyak and his false statements to the FBI, it was already known that Flynn was investigated as a possible unregistered foreign agent of the Turkish government. Flynn’s company, Flynn Intel Group, entered a $600,000 contract in August 2016 with a Turkish businessman with close ties to the Turkish government.

The contract was routed through the businessman’s Netherlands-based shell company, as The Daily Caller first reported.

Flynn registered as a foreign agent of Turkey on March 7, 2017. He is expected to testify at the trial of his former business partner, Bijan Kian, who was indicted as an unregistered foreign agent of Turkey. Turkish businessman Ekim Alptekin was also indicted, but he remains in Turkey.

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